Polyolefin particles and process for preparing the same

ABSTRACT

Polyolefin particles having a halogenated surface region, which are prepared by irradiating ultraviolet rays to an aqueous dispersion of polyolefin particles in the presence of a halogen gas. The polyolefin particles having a halogenated surface region have an improved property of retaining volatile blowing agents and can provide expandable particles having an improved storability in the open state.

This is a division of application Ser. No. 859,589, filed May 5, 1986,now U.S. Pat. No. 4,665,102 issued May 12, 1987.

BACKGROUND OF THE INVENTION

The present invention relates to particles of polyolefins the surfaceregion of which is halogenated, especially such polyolefin particlescontaining a blowing agent, and a process for preparing the same. Thepolyolefin particles having a halogenated surface region of the presentinvention are novel materials which have not been known from anyliteratures.

Hitherto, expanded resin articles rich in elasticity have been preparedby using polyethylene, polypropyrene or a mixture of polystyrene andpolyethylene as a raw material. However, expandable beads of theseresins containing volatile liquid or gaseous blowing agents are verypoor in retention of the blowing agents. Therefore, these resins cannotbe sold in the form of expandable beads unlike expandable polystyrenebeads, and have been sold as pre-expanded particles expanded once in apredetermined expansion ratio or as expanded articles.

It is an object of the present invention to provide polyolefin particleswhich can be impregnated with volatile blowing agents and which canretain the impregnated blowing agents for a long term.

A further object of the present invention is to provide expandablepolyolefin particles superior in retention of volatile blowing agents.

These and other objects of the present invention will become apparentfrom the description hereinafter.

SUMMARY OF THE INVENTION

It has now been found that the above objects can be achieved byhalogenating at least the surface region of polyolefin particles.

In accordance with the present invention, there is provided a particleof a polyolefin whose surface region is halogenated.

The polyolefin particles having a halogenated surface region of theinvention are prepared by dispersing polyolefin particles in water, andirradiating ultraviolet rays to the particles in the presence of ahalogen gas, thereby halogenating the surface region of the particles. Ablowing agent impregnated in the polyolefin particles can be retainedfor a long term. The impregnation of the blowing agent can be conductedbefore or after the ultraviolet ray irradiation.

DETAILED DESCRIPTION

The polyolefin resins which can be used in the present inventioninclude, for instance, low density polyethylene, high densitypolyethylene, linear low density polyethylene, polypropylene,ethylene-propylene random copolymer, ethylene-vinyl acetate copolymer,ethylene-styrene graft copolymer, ethylene-vinyl acetate-styrene graftcopolymer, chlorinated polyethylene, and the like. The polyolefin resinsmay be used alone or in admixture thereof. The polyolefin resins mayalso be used in admixture with less than 50% by weight of other resins.Further, the polyolefin resins may be those crosslinked by a knowncrosslinking technique, for example, by a peroxide. Chlorinatedpolyethylene which is uniformly chlorinated, is poor in retention ofblowing agents, but is improved by providing a barrier layer to theparticle according to the present invention.

Granulation of the polyolefin resins is not limited to a particularmethod and can be made by known methods. Typical method for preparingpolyolefin particles is pelletization by extrusion. The pelletizingconditions vary depending on the resins used, and are suitably selectedaccording to the resins used. The pelletizing conditions are notparticularly limited, but preferably they are selected so that theremaining strain of the obtained pellets is as small as possible. Also,the diameter of the particles is not limited. Particle size generallyadopted for foaming of beads is usable, e.g. 0.1 to 10 mm. Usually,particles having a particle size of about 1 mm are used.

Known volatile blowing agents can be used in the present inventionwithout any restriction. Examples of the volatile blowing agents are,for instance, a hydrocarbon such as propane, butane, pentane, hexane orheptane, a chlorofluoro hydrocarbon such as trichlorofluoromethane,dichlorodifluoromethane or dichlorotetrafluoroethane, a chlorinatedhydrocarbon such as methyl chloride or methylene dichloride, and thelike. The blowing agents may be used alone or in admixture thereof.Preferably, the blowing agent is impregnated in the resin particles inan amount of 3 to 30% by weight based on the resin. When the content ofthe blowing agent is less than 3% by weight, no desired expandableparticles are obtained. Good expandable particles are obtained withinthe range of 3 to 30% by weight, and impregnation in an amount of morethan 30% by weight is not required.

The degree of halogenation of the surface region of a polyolefinparticle is determined on the basis of the maximum halogen concentrationin the surface region. Preferably, in case of the particles having anaverage particle size of at least 0.5 mm, polyolefin particles arehalogenated to the extent such that the maximum peak in the distributionof the halogen concentration measured by an X-ray microanalyser appearsin the region between the surface and the 200 μm inner position of theparticle and the halogen concentration for the maximum peak is at least10% by weight (calculated from the ratio of the X-ray intensity to thatof a standard substance), preferably at least 15% by weight. Thestandard substance used in measurement of halogen concentration by X-raymicroanalyser is common salt palte (NaCl) when the halogen is chlorine,sodium bromide when the halogen is bromine, potassium iodide when thehalogen is iodine, and fluorite when the halogen is fluorine. When thehalogen concentration for the maximum peak is less than 10% by weight,the retention of a blowing agent is very bad.

Halogenation of only a very thin surface region of the polyolefinparticles is sufficient to improve the property of retaining blowingagents, and moreover it has the advantage that the improvement isachieved without deteriorating the properties that the polyolefin resinspossess originally. Halogenation of the whole particle to the inside isdisadvantageous, since it takes a long time when the granulatedpolyolefin resin is subjected to the halogenation and accordingly theproductivity is remarkably lowered. When a polyolefin powder before thegranulation is subjected to the halogenation, it is possible to conductuniform halogenation all over the individual powder, but such a workrequires much labor and increases cost. Like this, the halogenation upto the inside of the particle is not required essentially, but it is ofcourse permissible so long as the degree of halogenation is very low ascompared with that of the surface region and the properties that apolyolefin resin originally possesses are not substantially impaired.

Any of fluorine, chlorine, bromine and iodine can be used in thehalogenation. Chlorine is advantageous from the viewpoint of easiness inhandling and cost.

The polyolefin particles having a halogenated surface region of thepresent invention are prepared, for instance, by dispersing polyolefinparticles in water and irradiating ultraviolet rays to the particles inthe presence of a halogen gas, for instance, with introducing thehalogen gas to the aqueous dispersion of the particles, thushalaogenating the surface region of the particles. Certain kinds ofpolyolefin resins, e.g. low density polyethylene (density 0.920), can behalogenated even if ultraviolet rays are not utilized. The expandablepolyolefin particles of the present invention are prepared byhalogenating polyolefin particles in a manner as mentioned above andthen impregnating the obtained particles with a volatile blowing agent,or by dispersing polyolefin particles in water, impregnating theparticles with a volatile blowing agent, and then irradiatingultraviolet rays to the particles in the presence of a halogen gas,thereby halogenating the surface region of the particles. Theimpregnation can be conducted by known techniques.

The degree of halogenation can be controlled by changing the intensityand dose of ultraviolet rays, reaction temperature, reaction time, etc.,and they are suitably selected according to the kind of polyolefin resinused and the desired quality of the product so that at least the surfaceregion of a particle is chlorinated to form a barrier layer forpreventing a blowing agent impregnated in the particle from leaking.

The present invention is more specifically described and explained bymeans of the following Examples, in which all parts and % are by weightunless otherwise noted.

EXAMPLE 1

A pressure reactor equipped with a stirrer was charged with 100 parts ofa high pressure processed polyethylene (density 0.966, melt index 1.1)and 135 parts of water. To the reactor was added with stirring asolution or dispersion prepared by adding 0.45 part of dicumyl peroxideto 15 parts of water containing 0.05 part of an alkylbenzene sulfonatesurfactant (commercially available under the trademark "NEOPELEX" madeby Kao Atlas kabushiki Kaisha) at an elevated temperature with stirring.To the reactor was then added 50 parts of water containing 0.3 part ofcalcium phosphate, and after degassing, nitrogen was introduced into thereactor. The reactor was maintained at 100° C. for 2 hours and furtherat 140° C. for 4 hours, and was cooled to 40° C. The content was takenout, washed with water and dried, thus crosslinked polyethyleneparticles which were not fused together were obtained. The gel fraction(insoluble matter in boiling xylene for 48 hours) of the crosslinkedpolyethylene was 51%.

A glass reactor equipped with a stirrer was charged with 100 parts ofthe crosslinked polyethylene and 300 parts of water, and air in thereactor was replaced with nitrogen. The temperature of the system wasraised to 70° C., and at that time when reached 70° C., chlorine gas wasintroduced into the system at a rate of 0.5 part/minute, while uniformlyirradiating the system with a 100 W mercury lamp, to start the reaction.After conducting the reaction for 5 hours at 70° C., the irradiation andthe chlorine introduction were stopped, and the reaction system wascooled to 40° C. The content was taken out, washed with water and driedto give crosslinked polyethylene particles having a chlorinated surfaceregion.

The state of chlorination of the obtained particles was examined by anX-ray microanalyser. The region from the surface to the 100 μm innerposition of the particle had been chlorinated. The maximum peak for thechlorine concentration appeared at the outermost surface of theparticle, and the chlorine concentration thereof was 30%.

An autoclave was charged with 100 parts of the thus obtainedchlorinated, crosslinked polyethylene particles, 120 parts of a 0.5%aqueous solution of polyvinyl alcohol and 25 parts of butane. The systemwas maintained at 100° C. for 6 hours with stirring, and after coolingthe system, the particles were taken out and dried to give expandableparticles.

The change in property of retaining a blowing agent with the lapse oftime at 23° C. was examined in the open state.

The results are shown in Table 1 wherein the retention property for ablowing agent is shown by a weight percentage of the residual blowingagent to the initial amount of the blowing agent impregnated.

COMPARATIVE EXAMPLE 1

Expandable crosslinked polyethylene particles were prepared in the samemanner as in Example 1 except that the chlorination of the surfaceregion of the crosslinked polyethylene particles was not conducted.

The results are shown in Table 1.

EXAMPLE 2

A mixture of 100 parts of an ethylene-propylene random copolymer(density 0.90 g/cm³, melt index 9, ethylene content about 4.5%) and 0.05part of talc powder was extruded into pellets having a size of about 1.5mm. A glass reactor equipped with a stirrer was charged with 100 partsof the pellets and 300 parts of water. After replacing with nitrogen,the system was raised to 70° C. and at that temperature, thechlorination was started by introducing chlorine gas at a rate of 0.5part/minute with uniformly irradiating the system with a 100 W mercurylamp. After conducting the chlorination for 5 hours, the irradiation andthe chlorine introduction were stopped, and the pellets were taken out,washed with water and dried sufficiently to give ethylene-propylenerandom copolymer particles having the chlorinated surface region.

The state of chlorination of the obtained particles was examined by anX-ray microanalyser. The region from the surface to the 120 μm innerposition had been chlorinated. The highest degree of chlorination wasobserved at the outermost surface, and the chlorine concentration atthat position was 33%.

An autoclave was charged with 100 parts of the obtained particles, 120parts of a 0.5% aqueous solution of polyvinyl alcohol and 30 parts ofbutane. The system was raised to 120° C. with stirring and maintainedfor 6 hours at that temperature. After cooling, the particles were takenout of the autoclave and dried to give expandable particles. The blowingagent retention of the expandable particles was examined at 23° C. inthe open state.

The results are shown in Table 1.

COMPARATIVE EXAMPLE 2

Expandable ethylene-propylene random copolymer particles were preparedin the same manner as in Example 2 except that the chlorination of thesurface region was not conducted.

The results are shown in Table 1.

EXAMPLE 3

An autoclave equipped with a 100 W mercury lamp and a stirrer wascharged with 100 parts of linear low density polyethylene particles(density 0.920, melt index 2.1) having an average particle size of 1.5mm, 300 parts of water, 120 parts of a 0.5% aqueous solution ofpolyvinyl alcohol and 30 parts of butane. The temperature was raised to60° C. with stirring, and the autoclave was kept at that temperature for2 hours. After the lapse of 2 hours, chlorine gas was introduced intothe autoclave at a rate of 0.5 part/minute, while irradiatingultraviolet rays from the mercury lamp. The reaction was carried out for5 hours. After cooling the system, the particles were taken out, washedsufficiently with water, dehydrated and dried to give expandableparticles. The blowing agent retention of the expandable particles wasexamined at 23° C. in the open state.

The results are shown in Table 1.

EXAMPLE 4

The procedures of Example 1 were repeated except that the chlorinationwas carried out at 20° C. for 1 hour.

The state of chlorination of the obtained particles was examined, and itwas observed that the chlorination occurred in the region between thesurface and the about 50 μm inner position, and the chlorineconcentration for the maximum peak was 15%.

The results are shown in Table 1.

COMPARATIVE EXAMPLE 3

The procedures of Example 1 were repeated except that the chlorinationwas carried out at 20° C. for 30 minutes.

The state of chlorination of the obtained particles was examined, and itwas observed that the chlorination occurred in the region between thesurface and the about 30 μm inner position, and the chlorineconcentration for the maximum peak was 7%.

The results are shown in Table 1.

Is is observed in Table 1 that polyolefin particles whose surface regionis halogenated to the extent such that the miximum halogen concentrationis at least 10% have a remarkably improved retention property forvolatile blowing agents.

In addition to the ingredients used in the Examples, other ingredientscan be used in the Examples as set forth in the specification to obtainsubstantially the same results.

                                      TABLE 1                                     __________________________________________________________________________    Depth of   Maximum                                                                              Content of                                                                            Blowing agent retention                             chlorinated                                                                              chlorine                                                                             blowing agent                                                                         (Change in residual blowing                         surface    concentration                                                                        impregnated                                                                           agent with the lapse of time) (%)                   region (μm)                                                                           (%)    (%)     1 hour                                                                            1 day                                                                             7 days                                                                            20 days                                 __________________________________________________________________________    Ex. 1                                                                             100    30     25.9    96.1                                                                              90.0                                                                              80.1                                                                              70.1                                    Ex. 2                                                                             120    33     28.3    95.3                                                                              92.1                                                                              86  78                                      Ex. 3                                                                             80     25     27.3    91.0                                                                              87.2                                                                              76.5                                                                              70.1                                    Ex. 4                                                                             50     15     27.2    90.2                                                                              86.5                                                                              69.5                                                                              60.2                                    Com.                                                                              --     --     28.1    13.2                                                                              0   0   0                                       Ex. 1                                                                         Com.                                                                              --     --     26.3    16.4                                                                              2.1 0   0                                       Ex. 2                                                                         Com.                                                                              30      7     27.2    50.1                                                                              18.2                                                                              9.3 5.2                                     Ex. 3                                                                         __________________________________________________________________________

What is claimed is:
 1. A particle of a polyolefin whose surface regionis chlorinated to the extent such that the chlorine concentrationmeasured by an X-ray microanalyser and corresponding to the maximum peakin the region between the surface and the 200 μm inner position is atleast 10% by weight calculated from the relative X-ray intensity ratioto a standard substance.
 2. The particle of claim 1, wherein saidsurface region is chlorinated to the extent such that the chlorineconcentration measured by an X-ray microanalyser and corresponding tothe maximum peak in the region between the surface and the 200 μm innerposition is at least 15% by weight calculated from the relative X-rayintensity ratio to a standard substance.
 3. The particle of claim 1,wherein said polyolefin is at least one member selected from the groupconsisting of low density polyethylene, high density polyethylene,linear low density polyethylene, polypropylene, ethylene-propylenerandom copolymer, ethylene-vinyl acetate copolymer, ethylene-styrenegraft copolymer, ethylene-vinyl acetate-styrene graft copolymer andchlorinated polyethylene.
 4. The particle of claim 1, which has aparticle size of 0.5 to 10 mm.
 5. The particle of claim 1, which has aparticle size of 0.5 to about 1.5 mm.
 6. The particle of claim 1,wherein the thickness of the halogenated surface region is at most 120um.